High Voltage Detectors in Power Electronics
While prominent for AC lines, DC voltage detectors are hard to find and seldom used in power electronics especially for voltages higher than 1000V, voltages that are life-threatening. Why is this when HV DC rails are becoming popular in power electronics?
Voltage detectors (VD from now on) have the function of signaling the presence of voltage in a conductor within a system, mainly for safety reasons during research or when a service operation is going to be carried out. Most of the time the monitored conductors come from low impedance sources whose voltages and energies are considered life-threatening. In simpler words, voltage detectors make visible the invisible.
The US agency OSHA publishes a table of the reactions of the body when subjected to different currents.
|5mA||Subtle shock, but can let go of the object|
|10-30mA||Painful shock, freezing current, loss of control|
|50-150mA||Strong pain, strong muscular contraction, cannot let go|
|>1A||Cardiac arrest, burns, nerve damage|
Power Electronics and HV
Power electronics applications have been making its way into high voltage use, or medium voltage depending on the normative reference, enabled by new system topologies and semiconductors. Also new distribution grid architectures are laid out around medium voltage, be it AC or DC, to allow for new sources and consumers of energy. The need to develop new power electronic applications in medium voltage is backed up by research and development of high voltage semiconductors by different manufacturers and it has seen a steep rise in the last decade, be it diodes, thyristors, IGCTs, SiC Mosfets or IGBTs.
Indeed medium voltage DC is something that is becoming of increasing importance in modern networks as evidenced for example in photovoltaic energy, MV DC-DC converters, MV distribution grids or the IEEE 1709 Medium Voltage DC Power Systems on Ships standard.
Figure 1: Photo of the MVD15-detector
Although companies and research centres have more resources available when working with high DC voltages in power electronic modules to provide safety for users, it is not uncommon to see multimeters used as voltage detectors, for example at universities. This practice should be avoided since DMMs do not provide a clear indication, can be configured for AC or DC but not both, and are limited to 1000V.
|61243-1||Capacitive type to be used for > 1 kV||AC|
|61243-2||Resistive type to be used for voltages 1 kV to 36 kV a.c||AC|
|61243-3||Hand-held two-pole voltage detectors <1500V||AC/DC|
|61243-5||Voltage detecting systems that are single-pole and capacitively-coupled 1-52kV||AC|
|61243-6||Non-contact voltage detectors (NCVD) for use at nominal voltages above 1 kV AC||AC|
Traditionally voltage generation, distribution and use has been in AC. Thus equipments and installations have mainly needed AC voltage detectors and the development of these has been supported by the IEC 61243 standard. Following is a table that summarises them. The main conclusion from this table is that there are no standards for DC or combined AC/DC high voltage detectors, be it hand-held or fixed type detectors.
Since the use of high voltage DC is quite prominent these days both in power electronics and transmission and since HV clearly poses a life risk to researchers and service personnel it is not understandable why this type of device has been virtually ignored by the industry.
Figure 2: Photo of the MVD
Main Characteristics of VD
At Saker we recognized this problem and with our commitment to safety in handling high voltage equipment decided to develop a line of fixed type voltage detectors for use in power electronics and cabinets that work both on AC and DC and detect voltages >1000V. First we determined what the main characteristics of voltage detectors should be:
Clear indication. The presence of voltage must signalled in a clear way, either by a pulsating bright light, loud sound or both. Since fixed type VD are continuously monitoring the presence of voltage a loud sound would be annoying in practice and a bright blinking light is considered appropriate.
High reliability. The last thing you want to fail is a voltage detector as this is what a user will eventually trust, so VD must be protected against over voltages and be designed with ample safety margins. For example the lower the input current to the VD the better as this decreases internal power dissipation. One of our models, the MVD30 consumes less than 200uA even at its maximum DC input of 3500V, and can withstand large over voltages.
Autonomous. VD must be 100% independent of the system they are trying to monitor. A failure in the system in which they are part must not affect the VD at all.
Also important specifications of VD are size, activation voltage range, current draw, maximum continuous voltage, and over voltage withstand. Dielectric strength test voltages are also important since when installed in a power inverter a VD is now part of a larger high voltage system in which insulation coordination must be also met.
Saker designs and manufactures fixed type VD for power modules, cabinets or test platforms. They cover detection voltages from 20V to 40000V and there are two types, passive and active VD. Passive VD rely only on the current provided by the line it is monitoring, this is the case for example of the MVD series.
Active detectors have the advantage of a greater detection range. This need aroused for example when we were contacted by a client that wanted to detect for the presence of voltage in a MV AC line, but needed a fairly low activation voltage (50V) because capacitors were connected to this line and trapped charges could present a DC voltage in the line even when the AC line was switched off. The MVDZ detector provides a solution to this need, it is an active type detector for fixed installations. A high voltage insulator contains a string of high voltage resistors and detection electronics. A control box installed in a cabinet panel connects to the insulator, and signals if voltage is detected. Self-testing means are also carried out when this device is powered by a push-button. Being a high voltage detector partial discharges are important, and this type of detector is free from them due to the vacuum casting process and the very high 405 MOhm input impedance.
Figure 3: Photo of the MVDZ detector
The MVD series will suit most power electronic modules with small footprint, activation voltages starting at 20V and rated maximum up to 3500V. These modules have 4 high intensity LEDs that blink when voltage is present. There is also the option to install a plastic fiber optic cable to bring the light pulse to the panel of a cabinet.
Every researcher working with high voltages in power electronics is encouraged to increase safety while working with it. Voltage detectors are only a part of an integral safety system aimed at protecting the user from fatal shock but they provide the last line of awareness in detecting the presence or not of voltage.
About the Author
Luis Otero works Saker Medium Voltage.